The present invention relates to liquid chromatography apparatus, and more particularly to a syringe used by an autosampler to acquire samples of liquids.
In various analytical procedures, including liquid chromatography, a large number of liquid samples are processed sequentially in the same apparatus. An autosampler is used to obtain samples of liquids which are to be analyzed. The autosampler typically uses a syringe to acquire the sample. Performance of autosamplers is significantly influenced by the accuracy of sample acquisition and wear resistance of the syringe. Various configurations of syringes used to obtain liquid samples are known in the art.
An example of one type of known syringe is generally illustrated in FIG. 1. This prior syringe comprises a cylinder 10, having a first and a second end. The cylinder 10 is typically made of glass. The cylinder 10 has a bore hole 12 through its central portion which extends from the first end to the second end. A piston 14 which enters the bore hole 12 through the first end of the cylinder 10, is configured to slide in and out of the bore hole 12. A plunger 16 is attached to the piston 14 at an end portion thereof and is configured to be inserted for slidable engagement in the bore hole 12. The plunger 16 is typically made of Teflon. The area where the plunger 16 and the bore hole 12 come into contact creates a liquid tight seal. As the piston 14 is pulled out of the bore hole 12, the plunger 16 creates a vacuum which draws a sample into the bore hole. This necessitates that the bore hole 12 and the plunger 16 be fabricated within strict tolerances to achieve desired accuracy of sample.
A metal coupling 20 is disposed at the second end of the cylinder 10. A portion of the metal coupling 20 is threaded for attachment to mechanisms for initially receiving the acquired sample e.g., hose, needle (not shown). The metal coupling 20 has a Teflon seal 22 which serves to seal the connection between the glass 10 and the receiving mechanisms.
During the initial operation or process of collecting samples, undesired fluid such as gas bubbles or prior liquids may collect inside the bore hole 12. The presence of undesired fluid in the bore hole 12 can, among other things, adversely influence the accuracy of delivery of the syringe. In prior art syringes, it is a difficult task to remove the entrapped undesired fluid. To purge undesired fluid from the bore hole 12 the piston 14 and plunger 16 must be manually removed from the bore hole 12. Fluid may spill out and compromise the integrity and cleanliness of the fluid delivery system. Furthermore, removal of undesired fluid such as gas bubbles, typically cannot be done in an automated mode.
Additionally, in the prior art syringe illustrated in FIG. 1, the accuracy of the bore hole 12 is poor as its precision is limited by many factors in the manufacturing process. Present practice is to heat shrink a glass tube onto a wire mandrel. The wire mandrel diameter changes as it wears during extraction from the glass tube after cooling. The coefficients of thermal expansion vary from lot to lot and according to temperature variations so that producing a wire mandrel to a precision diameter is difficult. All of these factors result in an influence or potential variability of 1.22% in volume for a 250 microliter syringe. It would be very costly to reduce this influence because it would cause a high rejection rate to the vendor.
Another problem associated with the illustrated prior art syringe is that the plunger 16 on the piston 14 is influenced by friction with the bore hole 12. This friction can distort the plunger 16 by varying amounts dependent upon the coefficient of friction of the bore hole. An engineering estimate from finite element analysis indicates approximately 0.5% variability due to friction at 1 microliter injections. Still further, the Teflon seal 22 at the coupling 20 expands as the temperature rises, and because it is confined it has a tendency to yield. As the temperature of the Teflon seal 22 drops, the seal contracts, sealing pressure of the seal drops and the seal will leak. Also, if there is a long time period between draws to fill the syringe, the bore dries out and can influence precision by varying friction. Variability of friction can lead to premature wear.
Another prior art syringe is disclosed in U.S. Pat. No. 4,625,572 (the ""572 patent). The ""572 patent provides a cylinder pump for an automatic chemical analyzer or the like, which comprises a cylinder and a plunger. Both the cylinder and plunger are made of a rigid material. They are coupled together in a liquid tight sliding contact with each other without any elastic member such as an o-ring interposed between the sliding contact surfaces. Because the plunger and cylinder must be coupled together in a liquid tight sliding contact, both must be machined within strict tolerances. Machining the plunger and cylinder within strict tolerances is an expensive process.
The ""572 patent discloses the use of substantially the same material for both the cylinder and the plunger to maintain strict tolerances. This limits the effectiveness of the cylinder pump by necessitating the use of materials which are acceptable for both a plunger and a cylinder and may not be transparent. A compromise results in that materials can not be used which are ideally suited for use respectively as a plunger or a cylinder. The ""572 patent also requires that the contact surfaces of both the cylinder and the plunger be polished to a mirror-like finish. This further complicates manufacturing and increases the cost of the cylinder pump.
Furthermore, the ""572 patent provides no mechanism for removal of undesired fluid from the bore hole. Undesired fluid trapped in the bore hole can significantly reduce the accuracy of pumped volumes, and negatively affects the efficiency of the subsequent analysis of samples.
The present invention provides a fluid transfer apparatus having integrated end sealing which is inexpensive to manufacture, highly accurate and lasts significantly longer than previous fluid transfer devices.
According to the invention a fluid transfer device is provided for use in an autosampler. The fluid transfer device comprises a cylinder having integrated end seals sealing a displacement rod. The cylinder according to the invention is fabricated of a material such as Ultra High Molecular Weight (UHMW) plastic or the like which is rigid enough to minimize distortion of volume yet compliant enough to create a seal between itself and the surface of the displacement rod. The cylinder, has a first sealing end and a second sealing end, and is constructed as an integrated structure with a bore hole through its central portion, running from end to end. The diameter of the bore hole is larger than the diameter of the displacement rod. The bore hole according to the invention does not need to be machined to any special tolerances. The displacement rod is constructed of a rigid material and is dimensioned as a function of the volume of fluid that is desired to be displaced through the syringe. At the first end of the cylinder the diameter of the cavity decreases until the diameter of the cavity and the diameter of the displacement rod are substantially the same so as to form a compression seal between the rod and syringe.
The second end of the cylinder has an integrated externally threaded coupling configured to be attached to mechanisms for receiving the acquired sample, such as a needle or hose(s). As the displacement rod is withdrawn from the bore hole, a sample is drawn into the fluid transfer device. The volume of the sample drawn into the bore hole will be substantially the same as the volume of the displacement rod withdrawn from the bore hole. Additionally, a cross hole, for venting gas bubbles or other undesirable fluids (e.g. left over previous liquid(s)), is located on the displacement rod at a point so that it may be positioned inside the bore hole. The cross hole is connected to a passageway through the inside of the displacement rod leading to an opening on the surface of the displacement rod which, when the rod is in an appropriate position, leads outside of the cylinder. Thus undesired fluid inside the bore hole can be vented when the cross hole is appropriately positioned within the bore and a flow is induced by a slight positive pressure.
In an alternative embodiment according to the invention, a differential displacement configuration is provided wherein the inner diameters of seals disposed at extreme ends of a cylinder have different dimensions, to accommodate a displacement rod having different outer diameter dimensions. The displacement rod has two different outer diameters to allow very low volume samples to be drawn without requiring an unmanageably small diameter displacement rod.
Features of the invention include provision of a syringe having increased accuracy, lower cost and increased longevity. The entire cylinder portion can be fabricated as a unitary structure having external compression sealing which simplifies the manufacturing process, provides enhanced sealing and saves money. The syringe is configured with a bore hole inner diameter that is not critical thus saving the substantial cost and avoiding the complexities of manufacturing associated with maintaining precise tolerances. The seal created by the displacement rod and the cylinder wears more slowly than prior seals and is effectively retained by compressive forces exerted continuously on the exterior of the seal area. This results in a significant improvement in seal longevity over prior fluid transfer devices. In addition, the present invention allows the use of an automatic gas purge. By allowing for the automatic release of undesired fluid from the bore hole the present invention further increases accuracy over prior devices.